10 synthesis of reaction separation system lec 10 homogenous separation
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After separating the heterogenous mixture, attempt can then be made to separate the homogenous mixture produced from the reaction. This is a harder separation ! We need to create or add an extra phase in order to perform the separation. Remember from your earlier courses in separation ! Most common example : Distillation, Liquid Extraction, Super Critical Fluid Extraction etc... There used to be a preferred rule long practised by industry whereby distillation was given the first priority as chosen method for separating homogenous mixture. Now, it is used as a benchmark against other methods. Some guidelines to consider ! In sequencing the separator for homogenous mixture, a guide which can be used is to develop/design the sequence which consumes the least energy for the required separation. 1 2 Lecture 10 : SYNTHESIS OF SEPARATION SYSTEM – HOMOGENOUS MIXTURE SEPARATION
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Transcript of 10 synthesis of reaction separation system lec 10 homogenous separation
After separating the heterogenous mixture, attempt can then be made to separate the homogenous mixture produced from the reaction.
This is a harder separation !
We need to create or add an extra phase in order to perform the separation.
Remember from your earlier courses in separation !
Most common example : Distillation, Liquid Extraction, Super Critical Fluid Extraction etc...
There used to be a preferred rule long practised by industry wherebydistillation was given the first priority as chosen method for separating homogenous mixture. Now, it is used as a benchmark against other methods.
Some guidelines to consider !
In sequencing the separator for homogenous mixture, a guide whichcan be used is to develop/design the sequence which consumes the leastenergy for the required separation.
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Lecture 10 : SYNTHESIS OF SEPARATION SYSTEM – HOMOGENOUS MIXTURE SEPARATION
Extraction
Liquid-LiquidSolid-Liquid
ABSORBER
MEMBRANE
ADSORBER
2
Sequence preferred should bethe sequence that has highestenergy efficiency.
Eg. For distillation, assess the minimum vapour ffow required for the entire separation
if cannot performseparation oreconomically unattractive
What circumstances ? Discuss !
HOW ?
To illustrate the guidelines ….
DISTILLATION
First choice !
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Circumstances not favouring distillation
1. Separation of low molecular wt. materials
2. Separation of high molecular wt. heat sensitive material.
3. Separation of components with low concentration.
4. Separation of classes of components
Sequencing using minimum vapour flow
Use short cut method to determine minimum reflux ratio for specified separation
UNDERWOOD EQUATION
Comparison is based on a consistent basis : infinite no. of stages
Calculate minimum vapour flow for each required separation (as specified) in every alternative sequence. Go for the sequence with the least total minimum vapour flow.
DISTILLATION
DISTILLATION
DISTILLATION
DISTILLATION
DISTILLATION
DISTILLATION
calculate minimum vapour flowfor each column !
Use the Underwood Equation
We will look at this in more detail later …
Short Cut Design Method for Distillation Column
Stages of Calculation for the Short Cut Method.
i. Calculation of component distribution using Hengstebeck Method.
Cmb
dij
i
i
loglog
di
bi
Log (di/bi)
Log (aij)aHKaLLKaHHK
aLK
Log (di/bi) for LK
Log (di/bi) for HK
Stages of Calculation for the Short Cut Method.
ii. Calculation of Minimum Number of Stages using Fenske Equation.
Geometric average of the relative volatility between LK to HK at the top and bottom of column.
Minimum No of Stages
(Total Reflux)
LH
L
H
H
L
b
b
d
d
Nlog
.log
min
Another form of Fenske Equation
Stages of Calculation for the Short Cut Method.
iii. Calculation of Minimum Reflux Ratio using Underwood Equation.
qxNC
i ji
Fiji
11 ,
,,
NC
i ji
Diji xR1 ,
,,min 1
The equation is used to determine the root q which is then used to solve the minimum reflux ratio equation.
q – feed qualityai,j – relative volatility with reference to the heavy key jx i,F – feed composition , x i,D – Distillate composition
Stages of Calculation for the Short Cut Method.
iv. Determination of Feed Location
2
,
,..log206.0logHD
LB
L
H
S
R
x
x
D
B
z
z
N
NKirkbride Equation
ZH and ZL – mole fraction of heavy and light key respectively in feed.
XB,L and XD,H – mole fraction of light key in bottom product and heavy key in top product respectively.
B and D – molar flow of bottom product and distillate.
Ratio of the no. of stages in rectifying to stripping section.
Stages of Calculation for the Short Cut Method.
v. Determination of Actual No of Stages based on Set Reflux Ratio (Gilliland Correlation)
The correlation was originally represented in graphical form before an empirical was developed.
𝑋=(𝑅−𝑅𝑚𝑖𝑛 )
(𝑅+1 )
Based on the calculated Rmin from Underwood equation, the X value is determined based on a set Reflux Ratio R.
Rmin is determined from Underwood Eqn.
Then a Y value is determined from the X value using the correlation below;
𝑌=0.2788−1.3154 𝑋+0.4114 𝑋 0.2910+0.8628 ln 𝑋+0.9020 ln(𝑋+( 1𝑋 ))
𝑌=(𝑁−𝑁𝑚𝑖𝑛 )
(𝑁 +1 )
The Y value obtained is then used to determined the number of stages N corresponding to the Reflux Ratio R using the equation below based on the Nmin determined from Fenske Eqn.
Stages of Calculation for the Short Cut Method.
The correlation allows for the determination of corresponding No. of Stages N required as the Reflux Ratio R is varied from the minimum reflux ratio.
Reflux Ratio
No. of Stages x
x
x
x
xx
xx x x x
This will allow for the capital energy trade off to be investigated. (Discuss how it is being done?)
Example for Short Cut method calculation
Component Xfeed Distillate Flow Bottom Flow Mean Relative Volatil1ity
Propanei-Butanen-butanei-pentanen-pentane
Total
0.050.150.250.200.35
514.9241
0.4
45.3
00.1119
34.6
54.7
52.62.01
0.85
Given the separation specified below;
The separation cut is between n-butane and i-pentane
Determine the minimum number of stages and minimum reflux ratio.
Solution
Calculate the minimum no of stages using Fenske eqn.
8.82log
119
.124
log
min
N
Solution
Calculate the minimum Reflux Ratio using Underwood Eqn. (Assume sat. liq and therefore q = 1)
Component a i . zi a i . zi
a i - q
Propanei-butanen-butanei-pentanen-pentane
0.250.390.500.200.30
q = 1.5
0.0710.3551.0-0.4-0.462
0.564Sum
q = 1.3
0.0680.300.714-0.667-0.667
-0.252
q = 1.35
0.0680.3120.769-0.571-0.600
-0.022 Close enough, take q value = 1.35
qxNC
i ji
Fiji
11 ,
,,
i. Determine the root of Underwood equation q value.
Solution
Component x Di a i . xDi
a i - q
Propanei-butanen-butanei-pentanen-pentane
0.110.330.530.020.01
q = 1.35
0.150.691.66-0.06-0.02
2.42Sum = Rmin + 1
Therefore Rmin = 1.42
NC
i ji
Diji xR1 ,
,,min 1
ii. Using the calculated root of the Underwood equation q , determine Rmin.
Example on Distillation Sequencing
Component Xfeed Feed Flow Relative Volatil1ity
A - PropaneB - i-ButaneC - n-butaneD - i-pentaneE - n-pentane
Total
0.050.150.250.200.35
45.4136.1226.8181.4317.5
907.2 kmol/hr
8.093.492.681.231.00
Given the mixture to be separated;
Each component is to be separated with a specification of 99 %. Assume that the non key component will not distribute but end up either at the top or bottom depending on their relative volatility compared to the key component.
Determine the best sequence(s).
Example on Distillation Sequencing
The possible sequences are :
ABCDE
A/BCDE
B/CDE
BC/DE
C/DE
CD/E
D/E
C/D
B/C D/E
AB/CDE A/B C/DE
CD/E
D/E
C/D
ABC/DE
ABCD/E
AB/C A/B D/E
A/BC B/C D/E
A/BCD
A/BC
B/CD
B/C
C/D
AB/CD A/B C/D
BCD/E BC/D B/C
B/CD C/D
ABC/D
BC/D B/C
AB/C A/B
Example on Distillation Sequencing
For each of the column, we need to calculate the minimum vapour flow using Underwood Eqn….
ABCDE
A/BCDE
B/CDE
BC/DE
C/DE
CD/E
D/E
C/D
B/C D/E
AB/CDE A/B C/DE
CD/E
D/E
C/D
ABC/DE
ABCD/E
AB/C A/B D/E
A/BC B/C D/E
A/BCD
A/BC
B/CD
B/C
C/D
AB/CD A/B C/D
BCD/E BC/D B/C
B/CD C/D
ABC/D
BC/D B/C
AB/C A/B
The minimum vapour flow for each column in the sequence is added up to give the total minimum vapour flow for the sequence.
Example on Distillation Sequencing
Calculation done for some of the sequences…
ABC/DE AB/C A/B D/E
S Vmin = 5584 kmol/hr
ABCDE
ABC
DE
BC
A
B
C
D
E
Mixed Sequence
Example on Distillation Sequencing
Calculation done for some of the sequences…
A/BCDE BC/DE B/C D/E
ABCDE
C
DE
BC
A B
D
E
S Vmin = 5670 kmol/hr
Mixed Sequence
Example on Distillation Sequencing
Calculation done for some of the sequences…
ABCD/E AB/CD A/B C/D
S Vmin = 5106 kmol/hr
ABCDE
CD
AB
ABCD
D
C
A
B
E
Mixed Sequence
Example on Distillation Sequencing
Calculation done for some of the sequences…
ABCD/E ABC/D AB/C A/B
ABCDE D
ABC
ABCD
B
A
S Vmin = 5188 kmol/hr
C
E
CD
Indirect Sequence
In Summary…..
For homogenous mixture separation, distillation has always been preferred by industry due to their established position over other methods and their versatility /flexibility in addressing range of feed mixture.
As most reactor effluent mixture consist of many components, the separation for the individual components has to be done is a sequence. The sequence of these separators can be determined using established methods such as the short cut method (Fenske-Underwood Equation) for distillation column
